4.375 bore to a 4.46Bore

I've been running a BAE Stage 8 Hemi 4.375 stroke X 4.375 bore with a 18-71 DMPE M5 making around 49 lbs. Been running fairly well for years 4.11 1/8 on 10.5's at 2800 lbs. I run mild rpms with a 2 speed trans shifting at 9000 and cross the 1/8 at 9100

I've decide to increase my bore size to 4.446 and keep my 4.375 stroke crank with the same compression 11 to 1. Everything else kept the same also.

Question is should it make more power ?? I heard it will "un shroud the valve" my concern with the extra CI will it lose boost going from a 526 to a 548?

Boost isn’t everything. Sometimes you can be over boosted and it’s just as bad. Some Nhra cars aren’t even 40 psi of boost. All that number really is a what’s left over in the manifold according to Dmpe.

Increasing the bore will also increase your compression ratio also, do the math, This is how people get in trouble. An engine with a larger bore will always make more horsepower than one with a smaller bore, always. We just switched to a Hemi from a wedge style engine and "lost" 15 lbs of boost with the same blower. Guess what, we average 6 mph more at the 1/8th mile and have reset all of our personal best ET numbers (1\8th and 1/4).

Increasing the bore will also increase your compression ratio also, do the math, This is how people get in trouble. An engine with a larger bore will always make more horsepower than one with a smaller bore, always. We just switched to a Hemi from a wedge style engine and "lost" 15 lbs of boost with the same blower. Guess what, we average 6 mph more at the 1/8th mile and have reset all of our personal best ET numbers (1\8th and 1/4).

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of course I had the piston dome height reduced on the larger bore piston .500 down to .380 so the compression will remain the same with the larger 4.46 bore

I have a friend who ran IHRA TAFC and went from the 4 3/8 square motor to the 4.4675/4.150 deal and he said it gained 20 hp on the dyno and moved the power band up 200 rpm but wasn't really worth it. That was using Total Flow fatheads that were similar to BAE 5's with the 2.425" intake/1.95" exh. The newer heads have much larger valves so anything past either the BAE 8's or 8x's (Don't recall) will require the larger bore. The 4 3/8 stuff runs good but isn't as plentiful to buy used anymore. I still have some pistons I think but haven't seen much in the way of 4.375 cranks or pistons as of late. I think with the newer heads and bigger valves you may see a little hp gain but I doubt it would be more than 50 hp, Though you will need it in the future for upgrades if you want to get the bigger valve heads. As for the boost, all that boost is is air left in the manifold, trying to get rid of that boost without changing anything would mean the engine is using more air. Boost, Air temp, And hp to turn the blower at a given od all come into play to determine efficiency. MPH can also be an indicator. My PSI 2" setback HH retro on a 526 would make 49-50 lbs of boost with Total flow heads (BAE 5 like) at 43 percent od but took like 480hp to turn. My SSI F-3 made the same boost at 38 od but the outlet temps were cooler and it took a lot less hp to turn so was therefor a more efficient blower. My world record holding F-11 extreme unit will make less boost at 29 od but takes even less hp to turn and the outlet temps were even cooler yet so with some small compression added it set and still holds the world record. Hope that helped some, Trev

Staff Member

The reason you loose boost is a good thing - you have a bigger 'hole' to shove it in. Boost is not a true measure of airflow, it is a measure of back pressure in the intake. So if the blower remains the same, but the boost drops, well, you're moving more air. Theoretically it should make the same blower more efficient since it is going against less pressure.

You can get away with more timing with bigger bore, so there is definitely power there.